Tunable optical filters are useful devices for wavelength-division-multiplexing (WDM) systems, performing functions such as optical monitoring, channel selection in wavelength-based routing, noise filtering and coherent crosstalk reduction. As the number of wavelengths used in these systems grows, it is particularly desirable to have inexpensive tunable filters. Existing tunable filters have a relatively high unit cost, due to the labor-intensive fabrication and assembly processes which are used. Among these, tunable Fabry-Perot (FP) filters based on mechanical scanning of a FP cavity length are generally best suited to meet the high performance required in WDM systems, due to important optical properties such as, for example, low loss, polarization insensitivity, large tuning range and high bandwidth resolution. In addition to the problem of high cost, the tuning speed of bulk mechanical filters is typically rather slow, i.e., on the order of milliseconds, as the tuning process requires moving a relatively large mass.
A tunable FP filter is characterized by a cavity enclosed between two mirrors. The transmission function of a symmetric FP filter with identical mirrors is given by: ##EQU1##
where R is the mirrors' power reflectivity and .delta. is the accumulated phase a light wave acquires in each round-trip inside the cavity, given by: ##EQU2##
Here, n is the index of refraction of the material comprising the cavity (n=1 for air), L is the cavity length, and .lambda. is the operating wavelength. The resonant wavelengths of this filter are determined by the phase .delta. given above, and the separation between the wavelengths, called the free-spectral-range (FSR), is given approximately by: ##EQU3##
The passband width of the resonant peak is determined by the filter finesse F, which is a measure of the overall cavity and mirror losses: ##EQU4##
For an ideal lossless filter, the finesse is given by F=.pi.R/(1-R). The wavelengths that the filter transmits can be tuned, among other ways, by mechanically tuning the cavity length.
One type of conventional optimized tunable filter design approach sets the free spectral range to be about equal to the required tuning range. For WDM systems, a typical tuning range is in the range of 40-100 nm, and the center wavelength is approximately 1.55 .mu.m. Using Eq. (3), this translates to a cavity length of 10-30 .mu.m. If the WDM system uses 0.8 nm (100 GHz) channel spacing, a tunable filter used as a channel selector would require a filter bandwidth .ltoreq.0.5 nm, and from Eq. (4), a finesse of 80-200. This means that the required mirror reflectivity would be close to 98-99%.
In order to reduce cost substantially and also to enable faster switching speeds, micromachined FP filters have been developed. Examples of micromachined FP filters are described in M. C. Larson, and J. S. Harris Jr., "Broadly-tunable resonant-cavity light-emitting diode," IEEE Photon. Technol. Lett., Vol. 7, p. 1267, 1995; E. C. Vail et al., "GaAs micromachined widely tunable Fabry-Perot filters," Electron. Lett., Vol. 31, p. 228-229, 1995; P. Tayebati et al., "Widely tunable Fabry-Perot filter using Ga(Al)As-AlO.sub.x deformable mirrors," IEEE Photonic Technol. Lett., Vol. 10, pp. 394-396, 1998; J. Peerlings et al., "Long resonator micromachined tunable GaAs-AlAs Fabry-Perot filter," IEEE Photonic Technol. Lett., Vol. 9, pp. 1235-1237, 1997; A. Spisser et al., "Highly selective and widely tunable 1.55 .mu.m InP/air-gap micromachined Fabry-Perot filter for optical communications," IEEE Photonic Technol. Lett., Vol. 10, pp. 1259-1261, 1998; and U.S. Pat. No. 5,739,945 issued to P. Tayebati and entitled "Electrically tunable optical filter utilizing a deformable multi-layer mirror."
These micromachined filters share a common design approach, which defines vertically the entire FP structure, including its cavity and mirrors, by a sequence of multi-layer thin-film depositions on a wafer substrate. In this design approach, both top and bottom cavity mirrors are typically comprised of several quarter-wave-thick layers with alternating high and low refractive indices, while the layer which is used to define the cavity is a sacrificial layer which is later etched away in one of the final processing steps. The etching process forms a membrane or a cantilever structure. Cavity tuning is obtained electrically by pulling the membrane or the cantilever toward the substrate with electrostatic force, which changes the cavity spacing between the mirrors. Other types of filter designs, including horizontal cavity designs, are also possible.
In a typical mechanically-scanned micromachined tunable FP filter, the initial separation of the mirrors, and hence the exact starting wavelength position, i.e., rest position, of the filter passband in a given channel wavelength grid, is usually arbitrary and unknown until the assembly of the filter is complete. This poses a significant problem for system designers since a fixed set of applied tuning biases will sweep the filter passband through a tuning range, and thus a channel wavelength grid, which can differ from one device to another. Other problems with existing mechanically-scanned tunable FP filters include difficulties in controlling filter finesse, and difficulties in correcting for finesse degradation or other deviations from initial conditions over the operating lifetime of a given device. A need therefore exists for an improved micromachined tunable optical filter in which initial cavity spacing can be controlled independently of the tuning bias control, and in which finesse, wavelength position and other filter parameters can be more accurately controlled over the lifetime of the device.